The invention relates to lithography instruments used for patterning and processing substrates such as semiconductor chips and wafers. More specifically, the invention is concerned with a method for positioning stages during the processing of the substrates affixed to those stages.
In lithography processes the image from a reticle is transferred to a substrate, typically a wafer. System throughput is dependent upon the speeds of many separate steps that are performed in series. Throughput is therefore dependent on the duration of each step, which can include: loading, field image alignment, global alignment, and exposure.
The production of an acceptable final product requires the complex interaction of the systems necessary to implement each step. This complexity creates time requirements of its own. For example, when exposing patterns on wafers and other substrates, the reticle is moved at high speeds between discrete and precise positions to facilitate focusing the image on the substrate. This motion can generate dynamic reaction forces where the reticle is supported, leading to distortion of the reticle and, hence, distortion of the image focused on the substrate. Both reticle and wafer must be given time to settle to reduce the vibration that can cause distortion of the transferred pattern. Lithography processes typically occur in a clean room/vacuum environment; this is a source of further complexity and also an indication of the sensitivity of the processes.
A typical exposure apparatus employing a single wafer stage is shown in FIG. 1 and FIG. 2. Exposure apparatus 10 transfers a pattern of an integrated circuit from reticle 12 onto semiconductor wafer 14. Apparatus frame 16 preferably is rigid and supports the components of exposure apparatus 10. These components include: reticle stage 18, wafer stage 20, lens assembly 22, and illumination system 24. Alternatively, separate, individual structures (not shown) can be used to support wafer stage 20, reticle stage 18, illumination system 24, and lens assembly 22.
Illumination system 24 includes an illumination source 26 and an illumination optical assembly 28. Illumination source 26 emits an exposing beam of energy such as light or electron energy. Optical assembly 28 guides the beam from illumination source 26 to lens assembly 22. The beam illuminates selectively different portions of reticle 12 and exposes wafer 14. In FIG. 1, illumination source 26 is illustrated as being supported above reticle stage 18. Typically, however, illumination source 26 is secured to one of the sides of apparatus frame 16 and the energy beam from illumination source 26 is directed to above reticle stage 18 with illumination optical assembly 28. Where illumination source 26 is an electron beam, the optical path for the electron beam should be in a vacuum.
Lens assembly 22 projects and/or focuses the light passing through reticle 12 to wafer 14. Depending upon the design of apparatus 10, lens assembly 22 can magnify or reduce the image illuminated on reticle 12.
Reticle stage 18 holds and precisely positions reticle 12 relative to lens assembly 22 and wafer 14. Similarly, wafer stage 20 holds and positions wafer 14 with respect to the projected image of the illuminated portions of reticle 12. In the embodiment illustrated in FIG. 1 and FIG. 2, wafer stage 20 and reticle stage 18 are positioned by shaft-type linear motors 30. Depending upon the design, apparatus 10 may include additional servo drive units, linear motors and planar motors to move wafer stage 20 and reticle stage 18, but other drive and control mechanisms may be employed.
The basic device as described may be used in different types of lithography processes. For example, exposure apparatus 10 can be used in a scanning type lithography system, which exposes the pattern from reticle 12 onto wafer 14 with reticle 12 and wafer 14 moving synchronously. In a scanning type lithography process, reticle 12 is moved perpendicular to an optical axis of lens assembly 22 by reticle stage 18, and wafer 14 is moved perpendicular to an optical axis of lens assembly 22 by wafer stage 20. Scanning of reticle 12 and wafer 14 occurs while reticle 12 and wafer 14 are moving synchronously.
Alternatively, exposure apparatus 10 may be employed in a step-and-repeat type lithography system that exposes reticle 12 while reticle 12 and wafer 14 are stationary. In the step-and-repeat process, wafer 14 is in a constant position relative to reticle 12 and lens assembly 22 during the exposure of an individual field. Subsequently, between consecutive exposure steps, wafer 14 is consecutively moved by wafer stage 20 perpendicular to the optical axis of lens assembly 22 so that the next field of semiconductor wafer 14 is brought into position relative to lens assembly 22 and reticle 12 for exposure. Following this process, the images on reticle 12 are sequentially exposed onto the fields of wafer 14.
Processing a single wafer requires a significant time expenditure because of the complexity and sensitivity of the exposure apparatus and the steps involved. When a single wafer is undergoing one step, the apparatus for the others are normally idle. For example, when a single wafer is being exposed the apparatus for determining the alignment of the wafer relative to the wafer stage is typically idle. Consumer demand for the end product has created a need for increased throughput and, thus, the development of methods to decrease the idle time. A way to decrease idle time is to use two stages and position them so that each stage can undergo different steps of the process at the same time. The present invention is a method that uses two stages that run simultaneously, but with each stage at different steps in the process. This method relies upon a combination of encoders and interferometers to determine the position of each stage at any given point throughout processing. Encoders being rather less accurate than interferometers; the method preferably relies on them during the less position-sensitive steps of the process.
The present invention provides a two stage method where stage position may be determined using interferometers and one or more encoders. The stage assembly includes a plurality of interferometers mounted on a base for determining stage positions and encoders where interferometers are not feasible. The two stages move between multiple positions on the base and have mirrors affixed to them that cooperate with the other interferometer components to provide position data. At times, the two stages are positioned so that the first stage eclipses the second stage with respect to said at least one of the interferometers. Should such an eclipse occur, and another interferometer not be available for determining the eclipsed stage""s position, an encoder is configured to supply position. The apparatus is designed so that encoders are required during the less position-sensitive steps of the process, such as when switching from one step to another.
A method incorporating the invention comprises: sizing the stages based on wafer and exposure apparatus parameters; dispersing interferometers and encoders about the base at appropriate positions based on the stage sizes; moving the stages as desired while using the exposure apparatus; and determining the positions of both stages at all times during the process.